Autologous haematopoietic
stem cell transplantation has been tried as one experimental strategy for the treatment of patients with aggressive
multiple sclerosis refractory to other
immunotherapies. The procedure is aimed at ablating and repopulating the immune repertoire by sequentially mobilizing and harvesting haematopoietic stem cells, administering an immunosuppressive conditioning regimen, and re-infusing the autologous haematopoietic cell product. 'Non-myeloablative' conditioning regimens to achieve lymphocytic ablation without marrow suppression have been proposed to improve safety and tolerability. One trial with non-myeloablative autologous haematopoietic
stem cell transplantation reported clinical improvement and inflammatory stabilization in treated patients with highly active
multiple sclerosis. The aim of the present study was to understand the changes in the reconstituted immune repertoire bearing potential relevance to its mode of action. Peripheral blood was obtained from 12 patients with
multiple sclerosis participating in the aforementioned trial and longitudinally followed for 2 years. We examined the phenotype and function of peripheral blood lymphocytes by cell surface or intracellular staining and multi-colour fluorescence activated cell sorting alone or in combination with proliferation assays. During immune reconstitution post-
transplantation we observed significant though transient increases in the proportion of CD4+ FoxP3+ T cells and CD56(high) natural killer cell subsets, which are cell subsets associated with immunoregulatory function. CD8+ CD57+ cytotoxic T cells were persistently increased after
therapy and were able to suppress CD4+ T cell proliferation with variable potency. In contrast, a CD161(high) proinflammatory CD8+ T cell subset was depleted at all time-points post-
transplantation. Phenotypic characterization revealed that the CD161(high)CD8+ T cells were mucosal-associated invariant T cells, a novel cell population originating in the gut mucosa but expressing the central nervous system-homing receptor CCR6. Detection of mucosal-associated invariant T cells in post-mortem
multiple sclerosis brain white matter active lesions confirmed their involvement in the disease pathology. Intracellular
cytokine staining demonstrated
interferon γ and
interleukin 17 production and lack of
interleukin 10 production, a pro-inflammatory profile. Mucosal-associated invariant T cell frequency did not change in patients treated with
interferon β; and was more depleted after autologous haematopoietic
stem cell transplantation than in patients who had received high-dose
cyclophosphamide (n = 7) or
alemtuzumab (n = 21) treatment alone, suggesting an additive or synergistic effect of the conditioning regime components. We propose that a favourably modified balance of regulatory and pro-inflammatory lymphocytes underlies the suppression of central nervous system
inflammation in patients with
multiple sclerosis following non-myeloablative autologous haematopoietic
stem cell transplantation with a conditioning regimen consisting of
cyclophosphamide and
alemtuzumab.